Lets start with the in-boats. Since you run is salt water, I would go with a quality, true marine built speaker. They all have a different look and sound signature, so getting a demo can be helpful in making the choice thats right for you. Other then that, go with the ones that fit the budget and you like the look sand sound of. Here are a couple of key features for a marine speaker: stainless steel hardware, tinned speaker wire if any is included and an enclosed or insulated magnet. A few worth looking into are Kicker, Wet Sounds, JL Audio, Rockford Fosgate.
Amp: again due to salt water, I would suggest marine amps. The extra steps such as a conformal coated PCB, SS hardware, etc. Brand - again, lots of quality amps out there, so plenty of options. Kicker, Wet Sounds and JL Audio, Rockford Fosgate, Clarion all make a solid quality marine amp. I would suggest going with a more efficient amp like Class-D or G/H. Smaller and less battery current draw per watt RMS as compared to the older class-a/b.
One thing I see missing from your pan is a sub. Without a sub, you are missing out on a 4th of the music content. Just a small 10" sub with a little bit of power is all thats needed to fill in those missing lower bass notes. If you do decide to add a sub to the plan, then I would suggest a 5 or 6 chnl amp for the in-boats and sub. There are a handful that will do the job.
Tower setup: What are your tower speaker goals? Are you looking for a setup for strictly surfing and the party cove, a dedicated wake-board setup or a setup that will do both well? What we know what you are looking for in a tower setup, we can offer some suggestions.
Michael
Mikes Liquid Audio